Directional drilling is used to make wells or bore hole with non-vertical section and may be used for drilling wells or for directional boring, such as may be used to install utilities. In order to properly form the hole being drilled, measurement-while-drilling (MWD) systems are commonly used. An example of a MWD system is described in U.S. Pat. No. 6,088,294 (Leggett, III et al.) entitled “Drilling system with an acoustic measurement-while-driving system for determining parameters of interest and controlling the drilling direction” uses acoustic transmitters and receivers carried by the drill string to characterize the formation in which it is drilling. United States pregrant publication no. 2013/0118808 (Yang) entitled “Methods for geosteering a drill bit in real time using surface acoustic signals” analyses acoustic signature data to characterize the formation.
Communications with downhole equipment often presents difficulties. As the drill string is lengthened, any hardwired connections must also be extended, with additional time and effort. Alternatively, the signal may be sent wirelessly, however this has its own difficulties, as electromagnetic signals do not propagate well through ground.